1. Field of the Invention
This invention relates to a projection optical apparatus for projecting the pattern of a mask onto a semiconductor wafer for the manufacture of a semiconductor device such as an integrated circuit, and in particular to a projection optical apparatus provided with an inclination angle measuring system capable of measuring the absolute angle of inclination of the surface of the semiconductor wafer relative to the projection optic axis.
2. Related Background Art
Generally, in a projection optical apparatus used for the manufacture of integrated circuits, a projection objective having a great numerical aperture (N.A.) is used to form a minute circuit pattern image of the surface of a wafer and therefore, unless the exposed area of the wafer is maintained in a position accurately perpendicular (horizontal) to the optic axis of the projection objective having a very small allowable depth of focus and installed upright, a minute and clear-cut pattern image cannot be obtained. Particularly, in larger sizes of wafer and a new material such as gallium-arsenic replacing silicon, the planarity of the wafer itself is unstable and therefore, detection of the partial horizontal position of the wafer is required.
A horizontal position (level) measuring method is disclosed, for example, in Japanese Laid-Open patent application No. 113706/1983 (U.S. application Ser. No. 419,514, filed on Sept. 17, 1982) now U.S. Pat. No. 4,558,949. The method disclosed in this publication is a wafer surface inclination measuring system in which an off-axis and parallel light beam is applied to the surface of a sample, i.e., the surface of a semiconductor wafer and the reflected light therefrom is detected, whereby the average inclination in the exposed area relative to the optic axis is measured.
In this system, however, to find out the reference angle of inclination of the wafer, i.e., the angle of inclination of the wafer relative to the projection optic axis for which the sensor output becomes zero, it is necessary to repetitively effect the operation of effecting projection exposure processing for the wafer while changing the inclination of the wafer surface, thereby examining whether the projected pattern is uniformly well resolved over the entire projection range and effecting adjustment. Also, once the reference angle of inclination is determined, the exposure for the wafer must again be effected with respect to the relative change of the projection optic axis and the measuring system thereafter, and the adjustment thereof is cumbersome and very inconvenient and thus, means capable of making the most of the method which can determine the reference angle of inclination without effecting the projection exposure and applies an off-axis and parallel light beam to the surface of the smaple is desired.
On the other hand, a level measuring mehtod called the TTL auto leveling in which the inclination of the wafer is measured through a projection objective is also known. In this method, the contrast of a reticle mark projected onto a wafer is measured at four locations in the projection field, and chip leveling is effected with the surface on which the maximum value of each contrast is obtained as the optimum imaging plane. However, such TTL type level measuring method has the following inconveniences:
(1) The contrast is measured only at four points and therefore, it does not always represent the inclination of the entire surface.
(2) When the reticle mark is projected onto the wafer surface circuit, it is liable to be affected by the uneveness of the ground and thus, it is necessary to effect measurement while avoiding the uneven surface.